Add scss support for style property

[ovalkonia]

Description

Instead of applying the raw data at once in the :style widget property, it now feeds it to the grass::from_string function before applying, just as it's done in the :css property. It makes the usage of SCSS features possible, and since SCSS is compatible with CSS, there's no breaking changes.

Closes #1040

Usage

N/A

Showcase

Now this minimal configuration works just fine, which would not have gone so smoothly before due to the rgba(#FF0000, 0.4) format being a SCSS feature only!

Additional Notes

N/A

Checklist

Please make sure you can check all the boxes that apply to this PR.

• All widgets I've added are correctly documented.
• I added my changes to CHANGELOG.md, if appropriate.
• The documentation in the docs/content/main directory has been adjusted to reflect my changes.
• I used cargo fmt to automatically format all code before committing

[elkowar]

While there are technically some things in GTK-CSS that are incompatible with scss, I think they are uncommon enough that it's reasonable to assume it won't come up. I'll merge this, although in the unlikely case that people do rely on some niche GTK-css specific syntax somewhere in these style attributes, we'll need to revert back.